The present invention relates to a static magnetic field generator suitable for application to an IVR using a magnetic resonance imaging apparatus.
Magnetic resonance imaging apparatuses obtain a tomographic image of an examined portion of an object under examination using a nuclear magnetic resonance phenomenon. In order to use the NMR phenomenon, there is a need to locate the object under examination in a static magnetic field. In order to generate the static magnetic field, magnets are employed. Among the magnets employed for this purpose are permanent magnets, superconductive magnets, or resistive magnets, and according to the direction of the magnetic field, the magnets are categorized into the ones of a horizontal magnetic field type and a vertical magnetic field type.
In recent years, an Interventional Radiology (IVR) operation is sometimes performed. In this operation, while carrying out a diagnosis, medical treatment is performed, and the magnetic resonance imaging apparatus is sometimes employed for the IVR operation. The IVR operation is performed by directly touching the object under examination being diagnosed, so that openness is required. For the magnetic resonance imaging apparatus that satisfies this requirement, a magnet of the vertical magnetic field type is employed. A prior art that uses the permanent magnet of the vertical magnetic field type will be described first.
Among magnetic resonance imaging apparatuses that use the permanent magnet, which particularly take openness into consideration are the ones disclosed in JP-A-8-50170 and JP-A-11-104109. In JP-A-8-50170, permanent magnets vertically disposed to face each other are supported by two yokes brought to one side of the apparatus, thereby enhancing openness in a space where the yokes are not disposed. In JP-A-11-104109, permanent magnets are supported by a single yoke, thereby enhancing openness.
Among magnetic resonance imaging apparatuses that use the superconductive magnet, which particularly takes openness into consideration, is the one disclosed in JP-A-10-135027. In JP-A-10-135027, superconductive magnets disposed to face each other are supported by at least two supporting means brought to one side of the apparatus, and at the same time, a ferromagnetic memberic material that prevents leakage flux is also brought to one side of the apparatus, thereby enhancing openness in a side where the supporting means and the ferromagnetic memberic material are not disposed.
In the prior arts described above, in order to enhance openness, the yokes and the supporting means are brought to one side of the apparatus or formed to be a single one. With this arrangement, however, access to the object under examination during the IVR operation is limited. Specifically, in the configuration disclosed in JP-A-8-50170 or JP-A-10-135027, where the two yokes or the supporting means (referred to as two columns) are brought to one side of the apparatus for supporting, a bed is inserted such that the object under examination passes through between the two columns. Alternatively, the bed is placed at the side of the two columns and then inserted horizontally. Hence, when inserting the bed between the two columns, access to the object under examination is limited to the one from the front side of the direction of insertion. On the other hand, when inserting the bed from the side of the two columns, access is limited to the one from one side. Accordingly, there may be a case where the IVR operation is performed in an uncomfortable posture of the operator. Furthermore, the number of operators, to a certain extent, would be limited.
On the other hand, in the configuration disclosed in JP-A-11-104109, where a single yoke (to be referred to as a single column) is employed for supporting, the object under examination is placed at the side of the single column, and the frames of the yoke disposed above and under the permanent magnets have the shape of a rectangle. For this reason, when the IVR operation is actually performed, if access to the object under examination from the corner of the rectangle is made, a distance to the object under examination will be increased, thereby limiting the position of the operator for making access.
In view of the problems described above, the present invention has been made with an object to improve accessibility of an operator to an object under examination during an IVR operation.
In order to achieve the object described above, a magnetic resonance imaging apparatus according to the present invention comprises:
permanent magnets disposed in opposition to each other with respect to a space formed therebetween sufficient to accommodate an object under examination, for generating a static magnetic field in a direction of the opposition, a static magnetic field space being generated in the space;
pole pieces disposed on sides of the permanent magnets facing the space, for improving uniformity;
yokes disposed outside the permanent magnets to be opposed each other with respect to the space; and
a column for magnetically connecting the yokes;
wherein the column is formed with a single member,
wherein peripheries of the permanent magnets and the pole pieces are formed to be circular or generally circular, and
wherein the yokes comprise main body portions for mounting the permanent magnets thereto and protrusions joined to the column, and peripheries of the main body portions are formed to be arcs or like arcs so as to be aligned with the peripheries of the permanent magnets and the pole pieces. Further, the cross section of the column may be formed to be rectangular and has sides parallel to a line connecting the center of the column and the center of the static magnetic field, and the sides are made longer than sides perpendicular thereto. Still further, the column may be configured by laminating a plurality of ferromagnetic memberic plates. Further, the yokes may be thicker around the protrusions thereof for the column than in the portions thereof for the magnets.
Alternatively, a magnetic resonance imaging apparatus comprises:
superconductive magnets disposed being opposed each other with respect to a space formed therebetween sufficient to accommodate an object under examination, for generating a static magnetic field in the opposite direction, a static magnetic field space being formed in the space;
cryostats for surrounding the superconductive magnets, thereby cooling the superconductive magnets to a superconductive state and keeping the superconductive magnets in the superconductive state;
a connecting tube for coupling and supporting the cryostats;
first ferromagnetic member disposed outside the cryostats, being opposed each other with respect to the space; and
a second ferromagnetic member for magnetically coupling and supporting the first ferromagnetic member;
wherein the connecting tube and the second ferromagnetic member respectively comprise a single member. Depending on the circumstances, a cover may cover the connecting tube and the second ferromagnetic member to form as a single support structure. The peripheries of the cryostats are formed to be circular, the first ferromagnetic member comprises main body portions for mounting the superconductive magnets thereto and projections for being joined to the second ferromagnetic member, and the peripheries of the main body portions are formed to be like arcs so as to be aligned with the peripheries of the cryostats. Further, it is also possible to construct the apparatus such that the cross section of the second ferromagnetic member is formed to be rectangular and has sides parallel to a line connecting the center of the second ferromagnetic member and the center of the static magnetic field, and the sides are made longer than sides perpendicular thereto.
The second ferromagnetic member may be configured by laminating a plurality of magnetic plates. Further, the first ferromagnetic members may be thicker in the protrusions than in the main body portions thereof.
Still further, the bed can be rotated with respect to the center of the static magnetic field. The bed can be disposed so that the body axis direction of the object under examination is substantially oriented toward the center of the static magnetic field space, and can be disposed in a position where the body axis direction of the object under examination lying down thereon is off an extended line connecting the center of the static magnetic field space and the center of the column, and is also off a line that passes through the center of the static magnetic field space and is perpendicular to the extended line.
According to the present invention, the column or the second ferromagnetic member of the magnetic resonance imaging apparatus comprises a single member. Further, the peripheries of the main body portions of the yokes or the first ferromagnetic member is formed to be circular. With this arrangment, openness is improved, and accessibility of an operator to an object under examination is improved. Further, by making the sides of the section of the column and the second ferromagnetic member oriented toward the center of the static magnetic field space to be longer than the sides perpendicular thereto, openness and accessibility can be more improved. In addition, by deflecting the direction of inserting the object under examination on the bed from a line connecting the center of the column or the second ferromagnetic member and the center of the static magnetic field space, the column and the second ferromagnetic member do not hinder the operator from performing medical treatments, so that access can be made. Still further, by rotating the bed with respect to the center of the static magnetic field space, access can be made irrespective of the number of operators and the types of operations.